Savu config should not require a plugin to be executable to add it to a process list

[DanNixon]

There is no requirement for plugin dependencies to be present when creating a process list that uses a given plugin, so all plugins should be allowed to be configured in savu_config.

A note could be shown on those that cannot be executed on the current system.

This will be very beneficial for the webservice as the container size drops from ~5GB to just a ~100MB as no Savu dependencies are really required for process list manipulation.